1. Field of the Invention
The invention relates to an apparatus for generating azimuthally or radially polarized radiation by means of an optical waveguide, wherein the optical waveguide has a structure which is suitable for conducting azimuthally or radially polarized modes.
2. The Prior Art
Devices which emit radially or azimuthally polarized radiation are of great interest for a multitude of applications in the field of science, medical technology, military technology or civil engineering, for example for material machining, microscopy or for so-called optical tweezers, too. These applications call for light sources with azimuthally or radially polarized radiation, in particular simple, stable, powerful and cost-efficient sources with a high polarization purity.
In prior art, optical waveguides already exist, in particular glass fibres, which have a structure that is suitable for conducting various azimuthally or radially polarized modes. However, up to now it has not been managed satisfactorily to generate these modes stable and at small expenditure within the waveguide.
Only if the waveguide structure allows for higher modes it is possible for radially or azimuthally polarized modes to propagate within the optical waveguide. To be highlighted here in particular with rotation-symmetrical waveguide structures are the two TE01 and TM01 modes which, among others, have the property of an azimuthal and radial polarization. A problematic aspect herein is that modes with an approximately equal effective refractive index combine among each other so that a predominantly linearly polarized light beam is developed from the azimuthally or radially polarized modes. This is called a degeneration of modes. For example, in certain structures, the TE01 and TM01 modes combine with the HE21 modes to develop a light beam with a linear polarization. Waveguide structures admitting such combinations are called “weakly conducting”.
In prior art, there are various approaches to solve this problem. For example, with weakly-conducting multi-mode fibres, a radially polarized mode can be selectively excited by coupling a basic mode with an offset into the fibre (see T. Grosjean, D. Courjon and M. Spajer “An all-fiber device for generating radially and other polarized light beams”, Optics Communications, vol. 203, pp. 1-5, 2002). By means of a special fibre design, too, for example according to US 2009/0202191 A1, it is possible to generate a radially or azimuthally polarized mode. Herein, the fibre design enables a conversion of an already existing linearly polarized light beam. In combination with micro-bending and a polarization filter, the power of the basic mode within the fiber can be coupled into a radially or azimuthally polarized mode each.
Solutions known from prior art have a problem in that one cannot take recourse to standard components. Hereby, these are expensive and experimentally complex. The use of specially produced optical elements, each of which leading either to a radial or azimuthal polarization, is always mandatory. The necessary high expenditure on adjustments shows a direct influence on the polarization purity of the radiation. Therefore, often only a low polarization purity can be observed. Furthermore, weakly-conducting waveguides additionally entail a problem in that the azimuthally or radially polarized modes combine with other modes, thus reducing the polarization purity of the azimuthally or radially polarized beam.